Q: Thermostat stuck

If you are referring to the engine cooling system thermostat, if that thermostat is stuck closed or partly closed, your engine will overheat but such will be obvious upon reading the engine temperature gauge on the instrument panel. Obviously, if the car is overheating, based on what you see on that gauge, do not drive the car at all because of the potential for damage. If instead the thermostat is stuck open, the engine will NOT reach normal operating temperature and consequently you will experience "some" reduction in fuel economy and also, if such low temperature operation were prolonged, excessive condensation and deposits would be left within the engine. Short term operation with a stuck open thermostat is OK. however, as noted, if the thermostat is stuck closed, you can’t operate the engine at all. There are other temperature "sensors" on the engine, or in the radiator fan controls, and the effect of those being "stuck" or non operational will vary as to consequence. If the car is under warranty, they can, and should install an aftermarket thermostat while you await the OEM one that is allegedly on backorder. Obviously, a certified mechanic from YourMechanic can install an engine cooling thermostat for you as well. The service to request would be thermostat replacement. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.